To measure a concentration of an individual gas, the absorption of infrared radiation (IR radiation) of a certain wavelength range by the gas molecules of the individual gas in question over a measuring distance may be measured. For this purpose, an IR radiation source and a radiation detector, which is located opposite the IR radiation source over the measuring distance and may be accommodated in a sensor module, are used. In general, the radiation detector has plate-shaped filter elements which are applied over a thermopile structure having an absorber layer. The filter elements here determine the wavelength range and thus the individual gas to be measured. The infrared radiation passing through the radiation filter of the filter element strikes the absorber layer, where it is converted into heat which may be measured by the thermopile structure as a thermal e.m.f. The thermopile structure is usually applied to a diaphragm of a sensor chip and may be read by the sensor module via a contact. Sensor modules are known which have a metallic housing on whose bottom the thermopile chip is located, the filter plates being glued to the inside of the metallic cover of the metallic housing via openings.
Such a structure is, however, very complicated and expensive. Furthermore, the thermopile chip is not encapsulated, which makes the separation of the chip from the wafer using a sawing process and the assembly and handling of the chips very complicated and expensive. Furthermore, assembling the filter plates by gluing via openings made in the metallic cover is very complicated. To achieve proper adhesion, a filter plate is employed that is relatively large in relation to the optical range used, which is in turn expensive to manufacture due to its size. Furthermore, a considerable size of the filter plates is also necessary to even out adjustment tolerances between the base of the metallic housing having the thermopile chips and the metallic cover having the filters.
Due to the adhesive attachment of the filter chips to the metallic housing, the filter chips are no longer hermetically sealed at the adhesion points due to the welding of the hermetically sealed housing, so that for applications relevant with regard to safety in the automobile industry the wire contacts between the thermopile and housing contacts must be passivated. Passivation of the wire connections, which are used for contacting, between the thermopile chip located in the housing and the electrical contacts in the housing to protect them against external influences is generally not possible, because the passivating agent, usually a gel, would get into the path of the infrared radiation leading to the thermopile chip through the metallic cover having the radiation filter.
The radiation detector according to the present invention, the sensor module according to the present invention, and the method according to the present invention for manufacturing a radiation detector have the advantage over the related art that they make a more stable and reliable construction possible which has a relatively low complexity and low manufacturing cost, while meeting the requirements for use in the automobile industry.
In accordance with an example embodiment of the present invention, a thermopile chip manufactured using surface micromechanics having a cover chip is sealed hermetically, i.e., vacuum-tight, and a filter plate is glued directly onto this cover using an adhesive that is highly transparent in the relevant infrared wavelength range. An adhesive layer of a defined layer thickness is formed.
An additional chip having a filter structure formed on its top and/or bottom may be used in particular as a filter plate. The defined layer thickness may advantageously be achieved using spacer means, which may be, for example, spacers embedded in the adhesive layer, e.g., glass beads, glass rods, or defined granulates or crystallites or a structure of spacers formed on the top of the cover chip which do not affect the radiation path through the filter plate and the cover chip.
The chips are preferably made of silicon, which is highly transparent in the relevant infrared wavelength range; the use of silicon-germanium chips, among others, is also possible.
Due to the encapsulated structure of the radiation detector, the yield in the sawing process from the wafer(s) is improved, while handling and assembly are significantly simplified. The thermopile chips, cover chips, and optionally also the filter chips may be manufactured in different wafers, separated by sawing, and mounted on top of one another.
The connection between the thermopile chip and the cover chip is advantageously achieved via a bond connection, e.g., a seal glass bond, which forms a highly stable and vacuum-tight connection, while any mechanical tensions produced between the thermopile chip and the cover chip are negligibly small. The bond between the filter plate and the cover chip which is capable of transmitting the radiation and thus of fulfilling the function of the structure is formed by the defined adhesive layer.
The radiation detector according to the present invention may be inserted into a housing, preferably a premolded housing, by a standard assembly method. Manufacturing using a standard assembly method at a low cost is thus possible. Furthermore, no gluing process of filter plates into a metallic cover or adjustment to form a thermopile chip is needed. The direct chip-to-chip gluing of the filter plate onto the cover chip makes a simple gluing process and a simple adjustment of the filter plate possible.
According to the present invention, the filter plate may be selected to be very small, so that the manufacturing costs are kept low.
The encapsulated stack structure or layer structure according to the present invention makes it possible to glue the filter plate directly onto the cover chip using an adhesive layer, whereby advantageous integration of the radiation filter is possible without having to connect a filter to the seal glass bonds directly via the thermopile structure, which is not possible or is very expensive and would result in tension cracks in the filter layers due to the high temperatures for the seal glass process and the unsuitability of the filters for high temperatures.
The radiation detector according to an example embodiment of the present invention is encapsulated, i.e., the internal structure is in vacuum due to the hermetic seal glass bond; therefore, it may be inserted into a premolded housing, which may be designed as desired at a relatively low manufacturing cost. To passivate the wire bonds contacting the sensor chip, a passivating agent, for example, a gel, may be introduced, which does not affect the functionality of the encapsulated radiation detector.
Partial passivation of the wire bond may be achieved without contamination of the filter surface due to the enclosed chip structure and the great height between the bond area and the filter surface.
Furthermore, the testing complexity and testing costs may be kept low according to the present invention, because the function of the radiation detector may be tested in an open housing and repaired, for example, in the event of defective wire bonds.